Upper Back Injury Settlement Amount?

I had to have a disc removed from my upper back that was pressing against my spinal cord. It got dislocated on the job. I have been receiving $51 per month for 8 months titled Permanent-Partial disability. I went back to work for that company for almost another year before I decided to change jobs. My former employer has offered a lump sum payment of $18,000. Is that around the normal settlement and is it tax free? If anyone can help I would appreciate it. I'm new to this site.